GENOTYPIC VARIATION FOR GRAIN-YIELD AND GRAIN NITROGEN CONCENTRATION AMONG SORGHUM HYBRIDS UNDER DIFFERENT LEVELS OF NITROGEN-FERTILIZER AND WATER-SUPPLY

Sorghum [Sorghum bicolor (L.) Moench] is often grown under nitrogen- o r water-limited conditions, but there is little information on genotyp ic variation for grain yield and grain nitrogen (N) concentration unde r these conditions. This study examined the expression of specific ada ptation of hybrids to these stress conditions and, secondly, the effec t of N fertiliser application on yield and grain N concentration of th e hybrids. Two experiments, one irrigated and the other under rainfed conditions, were conducted in 2 seasons to examine 14 hybrids grown un der 3 levels of fertiliser N supply (0, 60, and 240 kg/ha). Genotypic variation for yield and grain N concentration was generally larger tha n the influence of genotypeXenvironment (predominantly N and water) in teractions. Genotypic variation for phenology was important in determi ning variation for yield and grain N concentration in high-input and r ainfed conditions when N was not the limiting factor, but not under N- limiting conditions. Under high-input conditions (240 kg/ha of N ferti liser and irrigated), maturity date accounted for about 50% of the gen otypic variation for grain yield (798-1049 g/m(2)), with later maturin g hybrids having a higher yield. Maturity date had little effect on pl ant N content at maturity or N harvest index, and hence grain N concen tration (1.67-2.01%) was negatively correlated with grain yield. Under N-limiting conditions, N fertiliser application had large effects on yield and/or grain N concentration in both well-watered and pre-anthes is water stress conditions. In the irrigated experiment, when N was li miting (0 and 60 kg/ha of N fertiliser), genotypic variation for grain yield (225-729 g/m(2)) was not related to that for maturity date. It was, however, related to the variation in N uptake and dry matter grow th by anthesis in the non-fertilised treatment. There was significant genotypic variation for grain N concentration (0.94-1.26%), which was not explained by variation for grain yield. Under rainfed conditions, where severe pre-anthesis water stress occurred, phenology was importa nt in determining about 40% of the genotypic variation for yield (69-2 86 g/m(2)). The late-flowering hybrids escaped the major impact of the pre-anthesis water stress, had reduced damage to panicle development, and-had higher N utilisation, consequently producing higher grain yie ld. Grain N concentration (1.09-2.85%) was again negatively related wi th grain yield. Genetic improvement of N uptake is identified as a pos sible breeding strategy for raising productivity and quality of grain sorghum under N-limiting conditions.